I think your being way too complicated.
US hills do not generally have extreme grades but you may want to venture off road or go to SF.
MTB style gearing will cope with most gradients.
You can't say which gear or gear inches you will need for a particular gradient, it depends on to many variable, the drag forces (surface/wind/load) and how you are feeling.
In my experience, steep starts at about 13% and killer climbs start at 18%.